This invention relates to the synchronization of periodic electrical signals and, more particularly, to the enhanced resolution of periodic electrical signals by synchronization and computer averaging. This invention is particularly useful for minimizing the effects of noise in electrocardiographic signals.
Many electronic devices utilize electrical, acoustical and/or electromagnetic signals that remain constant or vary in a specified manner. For example, electronic measurement devices, such as medical diagnostic apparatus, detect signals related to the physical variables of the system or its components being monitored. Some such devices also utilize transmitted signals in the measurement process. Also, electronic communication devices transmit and receive electromagnetic signals to and from locations where the information contained in the signals is to be utilized.
These devices frequently experience undesired disturbances within their useful frequency band. Such disturbances are commonly referred to as noise which denotes any unwanted fluctuations in the signal characteristics that are desired or expected to remain constant or to vary in a specified manner. Generally, noise results from either sources outside a circuit, in which case it is called interference, or from random or accidental fluctuations in the circuit itself due to motion of the current carrier. Since electronic noise is, by definition, an unwanted disturbance, its reduction in communication and measurement circuitry, as well as other types of circuitry, is a constant aim for both hardware and software engineers.
Two major classifications of noise exist with respect to signals. The first, additive noise, is a relatively uniform disturbance which is present throughout the entire signal. Types of additive noise include Johnson noise and shot noise. Johnson noise or thermal noise is the noise produced by thermal agitation of charges in a conductor. It is random and has a uniform energy versus frequency distribution. Johnson noise is random in that it contains no periodic components and its future value is completely unpredictable. Shot noise is also random and is exhibited by fluctuations of current output average value resulting from random emissions of electrons. Johnson and shot noise are both white, in that they have a constant energy per unit band width that is independent of the central frequency of the band. The second type, multiplicative noise, is not uniformly present throughout the entire signal but may recur at regular intervals. It is not a truly random noise. An example of multiplicative noise is that occurring in an electrocardiographic signal due to patient movement. Such noise is exhibited, for example, in recurrent amplitude fluctuations in the cycles of the ECG signal.
Electronic measurement devices are often utilized in the medical field to detect, display and analyze periodic or cyclical bio-electric and bio-acoustic signals emanating from a patient's body. These devices may passively receive only one type of signal or simultaneously receive heterogeneous types of signals, for example, electrocardiographic, phonocardiographic, radio emissive and NMR image signals. Reception of such signals may be made at one or multiple locations. Still other devices are utilized in the medical field to synchronize an actively transmitted signal such as ultrasound with a received signal for either diagnostic or therapeutic purposes. These electro-medical measurement devices have often incorporated hardware and/or software components to reduce the noise level in the signals.
One known method of noise reduction utilizes a computer to repeatedly average a plurality of recurrent signal cycles to yield a composite signal which is then displayed for operator diagnosis or further analyzed by other means. In the process of averaging, however, the noise components of each signal cycle are attenuated because they are weaker than the components desired for medical purposes.
Computer averaging of an electrocardiographic signal or other periodic electrical signals requires the alignment or synchronization of each signal cycle with respect to a fixed reference point so that an accurate comparison of corresponding portions or data points of the cycles may be made. The reference point is typically established in the time domain. In the past, synchronization of periodic signals with respect to a time reference was accomplished by establishing a single, fixed threshold voltage level, the attainment of which by each successive signal cycle provided a reference time coordinate for alignment purposes.
However, a problem exists in computer averaging because the signal noise sought to be mitigated by the averaging technique interferes with the synchronization technique. Specifically, additive noise in the threshold voltage portion of the cycle causes uncertainties in the determination of a reliable reference time coordinate from which to base alignment. And, improperly aligned signals greatly diminish the reliability of the averaged signal. Additionally, multiplicative noise can cause the failure of the determination of an alignment reference in some cycles. Without an alignment reference, these cycles are not synchronized, which results in further distortion of the averaged signal.
Yet another problem exists with regard to the synchronization of signals. Direct current (DC) shifts in an electrical circuit can cause the baseline to fluctuate or wander. The baseline or terrain of a signal is a region of least absolute value voltage or amplitude. The baseline corresponds to an area of inactivity or of least activity in the underlying physical phenomena represented by the signal. The baseline voltage may either be constant over a period in which case it is isoelectric or it may be variable. Baseline wandering results in inconsistent cycle amplitudes which do not realistically depict changes in the underlying physical phenomena that the signal represents. The resulting deceptive representation of such phenomena may lead to erroneous synchronization. DC shifts are caused, for example, by a loose electrode or by patient movement during the monitoring of electrocardiographic signals.
Despite the need for an effective method and apparatus for synchronizing noisy signals having wandering baselines for subsequent computer averaging, none insofar as is known has been proposed or developed. And, more generally, despite the need for an effective, general purpose method and apparatus for synchronizing signals having additive noise, multiplicative noise and/or DC shifts, none insofar as is known has been proposed or developed.
Accordingly, an object of the present invention is to provide a method and apparatus for establishing accurate reference points for waveforms having additive noise so that they may be synchronized with respect to time. It is another object of this invention to provide a method and apparatus for establishing accurate reference points for waveforms having multiplicative noise and varying amplitudes so that they may be aligned with respect to time. A further object of this invention is to provide a method and apparatus for establishing an accurate reference points for aligning signals or recurrent cycles of a periodic signal having a wandering baseline. It is a particular object of the invention to provide a method and apparatus for accurately synchronizing a periodic signal for its subsequent averaging to reduce noise. Finally, and more particularly, it is an object of the invention to provide an improved synchronization method and apparatus for computer averaging of electrocardiographic signals.